The invention relates to a drive assembly, especially for motor vehicles, of the type comprising an internal combustion engine and an exhaust gas turbine unit equipped with an exhaust gas turbine driven by the exhaust gases of the internal combustion engine.
Studies of the energy balance in a motor vehicle, performed on a water-cooled gasoline-powered internal combustion engine, produced an energy distribution wherein the energy supplied to the internal combustion engine by the fuel is expended as follows: approximately 35% with the exhaust gases, approximately 20% with the cooling water, and approximately 20% by heat radiation, so that only 25% useful internal combustion engine work acts on the crankshaft (FIG. 1 and Bosch Handbook of Auto Engineering, 18th Edition). From the above, it is clear that a considerable part of the fuel energy losses is attributable to the exhaust gases alone. This loss with the exhaust gases consists on the one hand of kinetic energy (exhaust gas exit velocity) and on the other hand of heat energy (high exhaust gas temperatures).
It is known that a portion of the theoretically achievable power is lost by the design-related incomplete expansion in reciprocating-piston internal combustion engines. This power loss L.sub.V, depending on the operating state of the internal combustion engine, is between 20 and 40% of the rated power L.sub.1 (FIG. 2).
Methods are known for reusing a portion of this energy. The best known such method is exhaust gas turbocharging. In this method, the exhaust gases drive an exhaust gas turbine mounted on a common shaft with a compressor. The compressor draws in fresh air, compresses it, and pumps it to the internal combustion engine. Other methods include the so-called compound system, wherein the power which can be provided by the exhaust gas turbine is fed back again to the drive shaft of the internal combustion engine, thus contributing to an increase in the efficiency of the internal combustion engine. However, the thermal energy of the exhaust gases remains unused in both methods.
On the other hand, it is known to use the heat energy of the exhaust gases for heating purposes (German Pat. No. 873,461) and to generate steam (German Auslegeschrift No. 1,157,430). The exhaust gases constitute an easily exploited heat reserve and are used for heating purposes unrelated to the operation of the internal combustion engine. However, this utilization of the heat energy has no effect upon the effective efficiency of the internal combustion engine. The steam generated by the heat energy from the exhaust gases can be used for steam turbines or steam engines, but still has no effect upon the effective power of the internal combustion engine.
The goal of the invention is to provide a drive assembly, especially for motor vehicles, wherein the amounts and types of energy residing in the exhaust gases can be utilized in such manner that the effective efficiency of the internal combustion engine is considerably increased by comparison with known drive assemblies.
This goal is achieved according to the invention by virtue of the fact that both the kinetic energy and the heat energy of the exhaust gases from the internal combustion engine are used by a thermal work process superimposed in the exhaust gas turbine of the exhaust gas turbine unit. The exhaust gas turbine unit comprises a compressor, a secondary turbine, and an exhaust gas turbine, whereby the turbine blades of the exhaust gas turbine are formed by hollow blades, through which the working medium compressed by the compressor flows, said medium expanding to release power in the secondary turbine. Since the kinetic energy of the exhaust gases is utilized directly by the exhaust gas turbine and the utilization of the heat energy of the exhaust gases is utilized indirectly by the exhaust gas turbine, with the latter operating as a heat exchanger and heating the working medium which has been compressed, a considerable recovery of power can be achieved in the exhaust gas turbine unit. This power can then either be fed directly back to the internal combustion engine or this power can be used to drive a generator for example, whose electrical energy can then be used to relieve the load on the internal combustion engine for performing a wide variety of duties, for example, driving accessories or for heating. In addition, the utilization of the heat energy of the exhaust gases according to the invention, by heat exchange between the exhaust gas turbine and the compressed medium, subjects the exhaust gas turbine unit to only slight thermal stresses. The working medium according to the invention can be air, steam, or the like, and the secondary turbine can consist of a single-stage or multi-stage axial or radial turbine. It has been found to be especially advantageous to make the exhaust gas turbine unit an integral unit. This results in the exhaust gas turbine unit occupying a small, compact volume, which is especially advantageous because of the small amount of available space, especially in motor vehicles.